An antenna duplexer shares a single antenna for transmission and reception, thus, is a type of a multiplexing circuit distributing transmission/reception signals, avoids external radiation and reception of spurious from transmission and reception bands, reduces external reception interference, and protects a reception side circuit at a time of transmission.
FIG. 1 shows a circuit configuration diagram of one example of a conventional antenna duplexer. In FIG. 1, to an antenna 1, one ends of distributed constant lines 2 and 3 are connected. The other end of the distributed constant line 2 is connected to a transmission port 5 through a transmission side bandpass filter 4. The other end of the distributed constant line 3 is connected to a reception port 7 through a reception side bandpass filter 6 (for example, the non-patent document 1).
When the antenna duplexer of FIG. 1 is designed, first the transmission side bandpass filter 4 and the reception side bandpass filter 6 are designed respectively, and then, the distributed constant lines 2 and 3 are designed respectively in such a manner that formulas (1) and (2) are met.
It is noted that ω01 denotes a center angular frequency of the transmission side bandpass filter 4, ω02 denotes a center angular frequency of the reception side bandpass filter 6, Yin1 denotes admittance viewed from the antenna 1 at the center angular frequency ω01, Yin2 denotes admittance viewed from the antenna 1 at the center angular frequency ω02, Re[ ] denotes a real part of the inside of the bracket, and Im[ ] denotes an imaginary part of the inside of the bracket.Re[Yin1]|ω=ω02=0,Im[Yin1]|ω=ω02=0  (1)Re[Yin2]|ω=ω01=0,Im[Yin2]|ω=ω01=0  (2)
It is noted that, in the patent document 1, it is described that a reception filter connected to a multiplexing circuit from an antenna includes a dielectric filer and a SAW filter connected thereto in a branching manner, and a transmission filter connected to the multiplexing circuit includes a dielectric filter.
Further, in the patent document 2, it is described that, a tap coupling type duplexer is used to form many attenuation poles at arbitrary frequencies.
Patent Document 1: Japanese Laid-Open Patent Application No. 10-41704
Patent Document 2: Japanese Laid-Open Patent Application No. 11-340706
Non-patent Document 1: K. Wada, T. Ohno, and O. Hashimoto: “A Class of a Planar Duplexer Consisting of BPFs with Attenuation Poles by Manipulating Tapped Resonators “IEICE Trans. On Electronics, Vol. E86-C, PP. 1613-1620 (2003-9).